350 
PALAEONTOLOGY OF NEW YORK. 
lamellae and become thickened at tlieir lower extension. The radii are 
stronger on the posterior part of the shell. 
The hinge structure has the usual characteristics of the genus. 
Four specimens measure respectively 35, 33, 23 and 21 mm. in length, and 
21, 16, 13 and 10.5 mm. in height. 
This species differs from M. Chemungensis in its wider posterior end, the 
absence of a constriction in the basal margin, more ovate form and the regular 
concentric varices. 
Formations and localities. This species occurs in the Hamilton group in most 
of the localities from Schoharie county to the centre of the State, but it is not 
very abundant. The greater number have been obtained in Onondaga county 
and at Canandaigua and Seneca lakes, N. Y. It has been observed in collec¬ 
tions from the Hamilton group, near Cumberland, Md. A form undistinguish- 
able from this species occurs in the soft shales at Bedford, Cuyahoga county, 0. 
Macrodon Chemungensis. 
PLATE LI, FIGS. 11-16. 
Macrodon Cliemungensin, Hall. Prelim. Notice LamellibrancMata, 2, p. 14. 1S70. 
“ “ “ Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 51, figs. 11-16. 
18S3. 
Shell of medium size or larger, arcaeform; cardinal and basal margins sub¬ 
parallel ; length more or less than twice the height; basal margin nearly 
straight or gently curving with a slight constriction anterior to the middle. 
Posterior end broad, sub-truncate or sometimes rounded. Cardinal margin 
straight, extremities angular. Anterior end rounded below, truncate above. 
Valves convex in the posterior portion, becoming gibbous in the middle 
and anterior. 
Beaks sub-anterior, prominent, rising a little above the hinge-line. 
Umbonal region gibbous, with a prominent, undefined ridge extending 
toward the post-inferior extremity, and a vertical depression extending from 
the umbo to the base. 
Surface marked by irregular, concentric, lamellose lines of growth. 
